Three methionine modified derivatives of ovine prolactin have been prepared: two by oxidation of the methionines by H2O2 to sulfoxide (partial and complete), and the third by complete alkylation of the methionines with iodoacetic acid to the carboxymethyl sulfonium salts. The derivatives were characterized by exclusion chromatography, amino acid composition, circular dichroism spectra, relative rates of digestion by trypsin, and biological activity. Partially oxidized prolactin, having four of its seven methionines oxidized, was very similar to the native hormone. The unmodified methionines in partially oxidized prolactin were found to be the residues at positions 36, 81, and 132. The prolactin derivatives in which all the methionines had been oxidized, or alkylated, showed major changes in all parameters examined. In addition, circular dichroism spectra indicated that complete modification of all the methionines in prolactin exposes the normally buried tryptophans. It is evident from bioassay data that oxidation of four methionines out of seven residues in ovine prolactin reduces slightly the biological activity. However, complete oxidation and alkylation of the methionine residues causes significant loss of prolactin activity. Highly purified canine prolactin has been prepared and partially characterized. The material is comparable in potency (11-27 units/mg) to ovine prolactin, and similar in amino acid composition as well. BIBLIOGRAPHIC REFERENCES: Li, C. H. Biological chemistry of pituitary lutropin. Memorial Volume to President Chiang Kai-shek, Academia Sinica, Taipei. Pp 257-287 (1976). Farmer, S. W., Papkoff, H., Bewley, T.A., Hayashida, T., Nishioka, R. S., Bern, H.A., and Li, C. H. Isolation and properties of teleost prolactin. Gen. Comp. Endocrinol. 31, 60-71 (1977).